The present invention relates to a multichannel device for measuring weak, varying magnetic fields of field strengths down to less than 10.sup.-10 T, in particular down to less than 10.sup.-12 T, the device containing in each channel a gradiometer of the first or higher order, formed by superconducting loops on a flat carrier element; a superconducting direct current quantum interferometer (DC-SQUID), rigidly joined to the carrier element mechanically; and superconducting connecting links between the gradiometer and the interferometer, including a coupling-in coil to couple the gradiometer signals inductively into the interferometer; as well as electronics for processing and displaying the signals generated at the interferometers of the channels. Such a measuring device is known from DE-OS 32 47 543.
The use of superconducting quantum interferometers, also called "SQUIDS" (abbreviation of "Superconducting QUantum Interference Devices") to measure very weak magnetic fields is generally known (See "J. Phys. E: Sci. Instrum.", vol. 13, 1980, pages 801/13 or "IEEE Trans. Electr. Dev.", vol. ED-27, No. 10, Oct. 1980, pages 1896/1908). Therefore, considered a preferred field of application for these interferometers is also medical technology, in particular magnetocardiography and magnetoencephalography because the magnetic fields generated by magnetic heart or brain waves have field strengths in the order of magnitude of about 50 pT and 0.1 pT, respectively ("Biomagnetism - Proceedings Third International Workshop on Biomagnetism, Berlin 1980", Berlin/New York 1981, pages 3 to 31).
The device for measuring such magnetic fields as described in the above mentioned DE-OS is of multichannel design in order to be able to determine a three-dimensional field distribution in short measuring times and, hence, sufficient coherence of the field data. Towards this end, each channel contains a gradiometer of the first or higher order, formed by superconducting windings of a sensor or detector loop and a corresponding compensating loop. The detector loops and compensating loops of the channels are combined into units each which are separated from each other in space. Consequently, the detector loop of a gradiometer is relatively far removed from the compensating loop associated with it. The still non-uniform, biomagnetic near field to be picked up selectively with these loops in the gradiometer area ("Rev. Sci. Instrum.", vol. 53, No. 12, Dec. 1982, pages 1815/45) is then coupled into an associated direct current quantum interferometer (DC-SQUID) via superconducting connecting links. Such SQUIDS, containing two Josephson contacts, are more sensitive than so-called radio frequency (RF) SQUIDS and have less of a characteristic noise signal. Since the gradiometers may be designed as coupling transformers, it is also possible by means of appropriate coupling-in coils to couple the magnetic flux inductively into the respective interferometer (see also "IEEE Trans. Magn.", Vol. MAG-17, No. 1, Jan. 1981, pages 400/3).
In the known device, the superconducting loops of the gradiometers of all channels are formed on a flat carrier element common to all. Also to be fastened to this carrier element is a carrier plate on which the interferometers of all channels and the associated coupling-in coils are formed. These coupling-in coils are connected to the respective gradiometer loops by superconducting connecting lines essentially running across the carrier element. This requires a costly contacting technique between the coupling-in coils and the connecting lines to be connected to them.
In addition, the known multichannel measuring device also contains electronics to process and display the signals generated at the interferometers of the channels, for which purpose normally conducting leads are connected to the plate supporting the interferometers at appropriate connecting points.
It is true that with such a construction the balancing problems generally inherent in a multichannel design can be controlled and extensive coherence of the field data can be assured. However, the active area of the system of gradiometer loops is relatively expansive and, because of the planar structure of the carrier element, generally not adapted to the surface contour of a patient to be examined. The cryostat required for the device to maintain the superconducting operating state of the superconducting components, therefore, must be accordingly large.